This invention relates to ceramic methods of coating substrates, and the substrates coated thereby, and more particularly to the low temperature formation of single layer and multilayer ceramic coatings containing aluminum nitride on various substrates.
It is desirable for electronic circuits, devices and other nonmetallic substrate materials to be serviceable under a range of environmental conditions. Further, many of the uses for electronic devices today place a premium on size and weight. For example, electronic circuits used in spacecraft, satellites, and military aircraft need not only to be able to withstand a wide variety of environmental conditions, but also must be compact and lightweight in use. In order to protect such devices and substrates from heat, moisture, ionic impurities, and abrasive forces, the art has resorted to a number of methods to coat the devices and substrates to prevent, or at least minimize, the exposure of the devices or substrates to these environmental conditions.
Early attempts at protecting electronic circuitry included potting the circuits in polymeric resins. However, these techniques added considerable thickness and weight to the circuits. Also, the polymeric coatings tended to absorb moisture from the environment which could eventually lead to damage or failure of the circuits. Presently, some circuits are contained in ceramic packages to protect them from environmental exposure. While the ceramic packages are relatively secure, they add a substantial amount of thickness and weight to the circuit. Further, they are relatively expensive to fabricate.
Others have applied passivating coatings to the surfaces of such substrates. Common causes for the failure of electronic devices is the formation of microcracks or voids in the surface passivation layer of the device, such as a semiconductor chip, permitting the introduction of impurities from the environment. For example, sodium (Na+) and chloride (Cl-) ions may enter electronic devices and disrupt the transmission of electrical signals. Additionally, the presence of moisture and volatile organic chemicals may also adversely affect the performance of electronic devices. A single coating material or layer may be insufficient to meet the ever increasing demands placed on the material by the electronics industry. Several coating properties such as microhardness, moisture resistance, ion barrier, adhesion, ductility, tensile strength, and thermal expansion coefficient matching must be achieved through the use of a number of thin protective layers on the electronic device.
More recently, lightweight single layer and multilayer ceramic coatings have been developed for coating electronic devices. For example, Haluska et al, in U.S. Pat. Nos. 4,753,855 and 4,756,977, teach the formation of ceramic coatings by producing a solvent mixture of a hydrogen silsesquioxane resin alone or in combination with a metal oxide precursor which is then coated onto the surface of an electronic device. The coating is ceramified at temperatures between about 200.degree. to 1000.degree. C. to form a silicon dioxide-containing ceramic coating. Additional coating layers of ceramic materials are also taught to provide additional protection and coating properties. These additional layers may comprise additional ceramic or ceramic-like coatings containing silicon, silicon and carbon, or silicon, carbon, and nitrogen.
The high refractory and chemically resistant nature of aluminum nitride coupled with other properties such as a large energy gap, a high thermal conductivity, and a closely matched thermal expansion to silicon make it an attractive prospective material for use in microelectronic packaging for the protection and passivation of electronic devices. While aluminum nitride has been prepared in powder form for the casting of larger parts, it has also been formed as a thin film by various chemical and physical vapor deposition procedures in the past.
For example, Tebbe et al, "A Thermoplastic Organoaluminum Precursor of Aluminum Nitride", Am. Ceram. Soc., Electronics Div., Denver (1987), teach the formation of an organoaluminum polymer from the reaction of triethylaluminum and ammonia which can be solidified, cured, and pyrolyzed to form aluminum nitride. Interrante et al, "Studies of Organometallic Precursors to Aluminum Nitride," Mat. Res. Soc. Symp. Proc., 73 Better Ceram. Through Chem. 2, pp. 359-66 (1986), teach the chemical vapor deposition of aluminum nitride using an organoaluminum amide intermediate. Others in the art have used reactive cathodic sputtering, glow discharge, vacuum deposition, or reactive ion beam deposition to form thin films of aluminum nitride.
A major drawback to these prior art techniques for forming thin aluminum nitride films is that they are relatively slow processes which require extended periods of time to build up even 1 to 10 micrometer layer thicknesses. Further, many of these prior art techniques must be carried out at very high temperatures, requiring the use of furnacing equipment and/or vacuum equipment. Additionally, such deposition techniques do not planarize or level the substrate surface but instead provide only conformal coverage of substrate surfaces, leaving discontinuities or thin spots in the coating.
Accordingly, the need still exists in the art for a simple, rapid, low temperature procedure for producing thin films of aluminum nitride on temperature sensitive substrates such as electronic devices, either alone or in combination with other protective layers.